Process and apparatus for controlled exposure of flexographic printing plates and adjusting the floor thereof

ABSTRACT

A method and apparatus to expose photosensitive printing plates with a predetermined radiation density from the main side (top) and a predetermined radiation density from the back side (bottom). The method comprises executing the main exposure with a time delay after the back exposure. The time delay between back exposure and main exposure is optimized to create smaller stable single dot elements on the photosensitive printing plate after processing and smaller single element dot sizes printed on the print substrate. The plate floor may be adjusted by performing a back-side-only exposure prior to executing the combined back and main exposure with the time delay.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT Application Ser. No.PCT/IB2016/001660, titled SYSTEM AND METHOD FOR CONTROLLED EXPOSURE OFFLEXOGRAPHIC PRINTING PLATES, filed 26 Oct. 2016, which claims priorityto U.S. Provisional Patent Application Ser. No. 62/246,276, filed on 26Oct. 2015. This application also claims priority to U.S. ProvisionalApplication Ser. No. 62/473,784, titled “PROCESS AND APPARATUS FORADJUSTING THE FLOOR OF A FLEXOGRAPHIC PRINTING PLATE IN A CONTROLLEDEXPOSURE SYSTEM OR PROCESS,” filed 20 Mar. 2017. All of the foregoingare incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Many processes are known in the art for preparing polymer printingplates, such as photopolymer flexographic plates and letterpressprinting plates coated with photopolymer material. One known processstarts with a plate having an ablatable material thereon, imaging theplate in a digital imager to ablate the ablatable material according toimaging data, and then curing the exposed plate by exposure of the plateto radiation, such as light energy, including but not limited toultraviolet (UV) light energy.

Various processes for curing the plate on both the imaged side and theback side of the plate by exposure to a functional energy source areknown, including methods for providing a blanket exposure (such as withfluorescent light tubes that emit UV light), and methods for providingthe desired radiation using light emitting diode (LED) technology, suchas is described in U.S. Pat. No. 8,389,203, assigned to the assignee ofthe present application and incorporated by reference. One particularlyuseful LED arrangement is shown and described in U.S. Pat. No.8,578,854, also incorporated herein by reference.

Known processes include exposing the back of a plate, then performinglaser ablation on the front side of the plate, then performing frontside exposure. Other processes include laser ablating the front side ofthe plate, then curing one side of the plate using a blanket exposure,manually flipping the plate, and then curing the other side of theplate. Each of the foregoing processes interposes an undefined, variabletime delay between the first and second exposure, depending upon theamount of time for the laser ablation step in the first process, ordepending upon the time it takes to manually flip the plate, in thesecond. This variability in elapsed time between first and secondexposure leads to undesirable variability in plate quality. Still otherprocesses may include exposing both the back side and the front side ofa plate simultaneously, which although it produces more predictableresults than a process that imposes a variable time delay, is still notoptimal, as discussed more herein later.

In the field of printing, minimizing the size of a dot printed on asubstrate is desirable, but smaller dots correspond to smaller printingplate elements, which are more susceptible to damage during use.Accordingly, there is always a need in the art to reduce the size orprinted dots, while also providing to optimal stability of the printingelements on the plate for making those printed dots.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic drawing depicting an exemplary apparatus for theback exposure of a photosensitive printing plate in accordance withaspects of the invention.

FIG. 1B is a schematic drawing depicting an exemplary apparatus, havingtwo front-side heads and one back-side head, for the back exposure of aphotosensitive printing plate in accordance with aspects of theinvention.

FIG. 2A depicts a “single element number 64” as referred to herein,comprising 8 by 8 single pixels.

FIG. 2B depicts a “single element number 144” as referred to herein,comprising 12 by 12 single pixels.

FIG. 3A is a photograph of a 3D perspective side view of an exemplaryprinting plate element.

FIG. 3B is a photograph of a top view of the exemplary printing plateelement of FIG. 3A.

FIG. 3C is a photograph of a top view of a dot printed on a substrate bythe printing plate element of FIG. 3A.

FIG. 4 is a table showing and depicting with photographs resulting dotdiameters corresponding to 64-pixel and 144-pixel single elementstructures exposed at various time delays from 0 to 1500 seconds.Numbers in this table use a comma as the decimal mark symbol to separatethe integer portion from the fractional portion of non-whole numbers.

FIG. 5 is a graph corresponding to the results of FIG. 4, illustratingthe dot ground diameter for the smallest processed single dot element ona printing plate versus time delay between back exposure and mainexposure for an exemplary set of processing conditions.

FIG. 6 is a graph corresponding to the results of FIG. 4, illustratingthe smallest printed dot diameter versus time delay between backexposure and main exposure for an exemplary set of processingconditions.

FIG. 7 is a schematic drawing depicting an apparatus having acylindrical configuration for the front and back exposure of aphotosensitive printing plate in accordance with aspects of theinvention.

FIG. 8 is a schematic drawing depicting an apparatus featuring a planarradiation source for the front and back exposure of a photosensitiveprinting plate in accordance with aspects of the invention.

FIG. 9 is a schematic drawing depicting an apparatus featuring a singlelinear radiation source for the front and back and front exposure of aphotosensitive printing plate in accordance with aspects of theinvention.

FIG. 10 is a flowchart depicting an exemplary method of the invention.

FIG. 11 is a schematic illustration depicting a flatbed embodiment ofthe invention.

FIG. 12 is a schematic illustration of a light source embodimentcomprising a plurality of units having a plurality of rows of pointsources.

FIG. 13 is a schematic drawing depicting a portion of the plate duringexposure.

SUMMARY OF THE INVENTION

One aspect of the invention comprises an apparatus for exposing aprinting plate, the printing plate comprising a photosensitive polymeractivated by exposure to radiation, the printing plate having anon-printing back side and a printing front side with a mask fordefining an image to be printed. The apparatus comprises one or moreradiation sources collectively arranged to expose the front side and theback side of the printing plate to radiation, a holder configured toreceive the printing plate in a position to receive incident radiationfrom the one or more radiation sources, and a controller connected tothe one or more radiation sources. The apparatus is configured to first,adjust floor thickness of the printing plate by providing one or moreback-side-only exposure steps, and then, for each specific coordinatecorresponding to a cross-sectional portion of the plate, first commenceirradiating the back side of the plate, then automatically impose aprecisely defined and repeatable time delay, and then immediately afterthe time delay elapses, commence irradiating the front side of theplate, all without exposing any specific coordinate to front side andback side irradiation simultaneously. Some embodiments comprise at leastone front source and at least one back source of radiation, wherein theat least one front source is positioned to expose the front side of theplate, and the at least one back source is positioned to expose the backside of the plate. In some embodiments, each of the at least one frontsource and the at least one back source have an irradiation fieldcovering an area at least coextensive with a width of the plate but notcoextensive with a full length of the plate, wherein the front and backsource are spaced apart from one another by a lateral distance along thelength of the plate. Such embodiments further comprise means for causingrelative movement between the printing plate and the front and backsources, wherein the relative movement has a velocity sufficient tocause the defined time delay over the lateral distance. In suchembodiments, a trailing edge of the back source may be spaced apart by alateral distance along the length of the plate from a leading edge ofthe main source. In embodiments in which the printing plate is fixed,the means for causing relative movement comprises means for moving thefront and back radiation sources relative to the substrate. Inembodiments in which the front and back radiation sources are fixed, themeans for causing relative movement comprises means for moving thesubstrate relative to the sources of radiation. In some embodiments, thesubstrate is cylindrical and the velocity is a rotational velocity ofthe cylinder.

In other embodiments, the plate, the at least one front source, and theat least one back source are all stationary, and the controller isconfigured to implement the time delay by imposing a time differencebetween activating the at least one back source and activating the atleast one front source, including in an embodiment in which each of theat least one front source and the at least one back source each areconfigured to emit a radiation field covering an area at leastcoextensive with both a length and width of the plate. Still anotherembodiment comprises a single source of radiation and means fortraversing the single source around a stationary printing plate.

Another aspect of the invention comprises a process for exposing aprinting plate comprising a photosensitive polymer activated by exposureto radiation, the printing plate having a non-printing back side and aprinting front side with a mask for defining an image to be printed. Theprocess comprises the sequential steps of (a) commencing irradiating theback side of the printing plate; (b) automatically imposing a preciselydefined and repeatable time delay; and (c)

immediately after the time delay elapses, commencing irradiating thefront side of the printing plate, without exposing any specificcoordinate to both front side and back side irradiation simultaneously.The method further comprises adjusting floor thickness of the printingplate by providing one or more back-side-only exposure steps prior toperforming step (a). In an embodiment in which each radiation step isonly a fraction of a total amount of desired radiation, the methodcomprises repeating steps (a) through (c) until the plate has beenexposed to a desired amount of total radiation.

In a process for optimizing the time delay for a specific type ofprinting plate at a specific set of exposure conditions, the methodcomprising performing steps (a) through (c) for a plurality of samplesof a specific type of plate for a specific set of exposure conditionsfor a plurality of different defined time delays, creating a pluralityof prints, each print corresponding to one of the plurality of samples;and selecting as an optimum the time delay corresponding to the printhaving the smallest stable print dots. An exemplary method foridentifying the optimum time delay may comprise performing severalexposure samples at different time delays, with an identical number ofexposure repetitions, back and front irradiation, then printing all ofthe samples and selecting the time delay corresponding to the printwhich holds the smallest print dots. In some instances, the step ofadjusting the delay time may comprise selecting a time delaycorresponding to a minimum value for the smallest printed minimum dotdiameter that coincides with a maximum value for the dot ground diameterfor a range of time delay values.

The process may be performed using any of the apparatus describedherein. Thus, in a process performed using an apparatus having at leastone stationary front source positioned to irradiate the front side ofthe plate, at least one stationary back source positioned to irradiatethe back side of the plate, and a stationary plate, the method comprisesimplementing the time delay by imposing a time difference betweenactivating the back source and activating the front source. In a methodperformed using an apparatus comprising at least one front sourcepositioned to irradiate the front side of the plate, at least one backsource positioned to irradiate the back side of the plate, each of theat least one front source and the at least one back source having anirradiation field covering an area at least coextensive with a width ofthe plate but not coextensive with a full length of the plate, and theat least one front source and the at least one back source spaced apartfrom one another by a lateral distance along the length of the plate,the method further comprises the step of causing relative movementbetween the printing plate and the at least one front and at least oneback sources, wherein the relative movement has a velocity sufficient tocause the defined time delay over the lateral distance.

In an embodiment in which the printing plate is stationary, the step ofcausing relative movement comprises moving the at least one front sourceand the at least one back source relative to the substrate. In anembodiment in which the at least one front source and the at least onefront back source are stationary, the step of causing relative movementcomprises moving the substrate relative to the radiation sources.

Yet another aspect of the invention comprises a printing plate preparedby any of the methods or using any of the apparatus described herein, inwhich the printing plate has stable print dots smaller than a plateprepared using a method that does not include the defined time delaybetween commencement of back side and front side irradiation. The delaytime may be optimized such that the curing result after completeprocessing of the printing plate yields smaller and more stable dotsfrom the printing of the plate as compared to a plate exposed withoutsuch a time delay or compared to a plate exposed with a very long timedelay.

DETAILED DESCRIPTION OF THE INVENTION

Those of skill in the art understand that oxygen is distributedthroughout the photopolymer resin of a polymer plate at the time it istypically processed, and that oxygen is an inhibitor of thepolymerization reaction commonly harnessed for curing the plates.Although polymerization caused by exposure of the polymer to actinicradiation scavenges this distributed oxygen, ambient oxygen will diffuseback into the resin over time if the plate is in contact withatmospheric air. Surprisingly, in processes in which a back exposure andmain exposure are both performed on a plate, it has been found that finedetail on a plate may be optimized by imparting a defined delay betweenperforming the back exposure and the main exposure. Without being heldto any particular mechanism, it is believed that in this defined delaytime following the back exposure, which scavenges oxygen from the backportion of the plate, oxygen from the front side of the plate starts todiffuse to the back side, thus creating a slightly less oxygen richconcentration in the area of the plate nearest the floor of the plate,such that the polymerization reactions near the floor of the plate reactfor longer before stopping and therefore create shapes on the plate thattaper from the floor toward the top of the plate following the mainexposure. It should be noted that a delay that is too long will resetthe entire plate to being oxygen saturated, and a delay that is tooshort may not permit sufficient oxygen diffusion to produce optimalresults. Thus, while the amount of the optimal delay may vary dependingon any number of characteristics, what is important is that the delaynot be too long or too short, for optimal results. This delay may beimparted in any number of ways, described in more detail herein.

An exemplary apparatus 100 for the back exposure of photosensitiveprinting plate 130 is shown schematically in FIG. 1. As is well known inthe art, printing plate 130 comprises a photosensitive polymer 134 onwhich is disposed a mask 132 that defines portions of the plate that aremasked from radiation exposure relative to portions of the plate thatare desired to receive such exposure. In a typical embodiment, thepolymer 134, including in the mask 132 area, is permeable to oxygen.

In apparatus 100, a UV source of actinic radiation 120 with apredetermined power density is scanned at a specific speed (v) under thebottom of the plate. For the main or front exposure of thephotosensitive printing plate a second UV source of radiation 110 with apredetermined power density (irradiance) is scanned above the plate withthe same specific speed (v). UV sources of radiation 110 and 120 areconfigured to scan the printing plate with the same speed (v). Such aconfiguration may be provided by synchronizing sources 110 and 120 tohave a same speed using a controller, or both sources may be attached toa common carriage that traverses the plate, with sources 110 and 120spaced apart from one another a suitable distance in the direction ofcarriage travel to provide the desired delay when the carriage moves ata predetermined speed. The predetermined irradiance may be the same forthe main side and the back side, or may be different. Preferably theirradiance at the rear side is only a fraction of the irradiance of thefront side exposure. Typically, the irradiance at the rear side is in arange of 10% or less of the front side irradiance, but the invention isnot limited to any particular ratio of front to back irradiance. Thepredetermined irradiance is typically a function of the characteristicsof the specific type of plate to be exposed, as is known to those ofskill in the art, and as is dictated by the manufacturers of suchplates.

The time delay between the back exposure with UV source of radiation 120and the main exposure with UV source of radiation 110 may be adjusted bythe control system 140 by adjusting the speed of the sources and/ormechanically by setting a constant distance (D) between the sourcesduring the scan process. The time delay t=D/v. Thus, mechanicallyvarying D has an impact on the delay, as does the relative speed betweenthe plate and the sources during exposure. The time delay can beoptimized to get smaller single dot elements on the photosensitiveprinting plate after processing and smaller single element dot sizesprinted on the print substrate. It should be understood that thearrangement depicted in FIG. 1 is schematic in nature only, to show therelationship between the light sources and the distance D relative to aplate. In a system 100 in which printing plate 130 is disposed along ahorizontal plane (i.e. in which directional arrow Y of the X-Y axisshown in FIG. 1 represents the directional pull of gravity), plate 130may be mounted on a transparent substrate 160 (such as glass). In asystem 100 in which printing plate 130 is disposed along a verticalplane (i.e. a system in which directional arrow X of the X-Y axisrepresents the directional pull of gravity), the plate may be hungvertically (such that no substrate under the plate or other structurebetween the radiation source and the plate are required), such as from ahanger 170. It should be understood that hangar 170 as depicted in FIG.1 is intended only to be schematic, and is not intended to represent anyparticular hangar geometry. Furthermore, although shown in a flatorientation, it should be understood that the printing plate may beflexible enough to be disposed around a transparent cylinder, such as aglass cylinder, or the plate may be in the form of a continuous sleeve,as is known in the art, with the distances between the light sourcesarranged relative to the rotational direction of the cylinder, asgenerally depicted in FIG. 7 and described in more detail herein later.

The relative movement between the radiation sources and the plate may beprovided by any mechanism known in the art for moving objects relativeto a horizontal, vertical, or otherwise disposed stationary surface. Forconfigurations in which the radiation sources move and the plate isstationary, for example, the sources may be disposed on a gantry systemhaving arms that pass the respective sources above and beneath astationary horizontal plate mounted on a substrate configured to permita sufficient amount of radiation to pass through, or on either side of avertically mounted plate. For configurations in which the radiationsources are stationary and the plate is movable, for example, the platemay be mounted on any mechanism known in the art, such as a movablestage configured to move relative to fixed sources on opposite sides ofthe stage. Mechanisms for rotating a cylinder on which a plate ismounted relative to fixed sources are well known in the field ofprinting. Similarly, mechanisms for rotating sources relative to a fixedcylinder on which a stationary object is mounted are also well known,such as in the field of medicine (e.g. CAT scan machines). Thus,mechanisms for moving one or more elements relative to another are wellknown in the art, generally, and the invention is not limited to anyparticular mechanism.

As shown in FIG. 1, it should be understood that each of the frontsource 110 the back source 120 have an irradiation field covering anarea at least coextensive with a width of the plate (wherein the “width”lies along the third dimension not shown in the 2-dimensional image ofFIG. 1) but not coextensive with a full length of the plate (wherein the“length” lies along the X-axis as shown in FIG. 1). Each of the frontsource and the back source may thus comprise a linear source (such assources 1120 and 1120 shown in FIG. 11) that emits radiation along aline parallel to the width of the plate. Each linear source, however,may comprise a plurality of subsources (such as LED point sources 1112shown in FIG. 12) that together collectively create the linear radiationfield having a defined length less than the length of the printingplate, and a width that spans at least the entire width of the printingplate.

In one embodiment, shown in FIG. 11, carriage 1130 may comprise a firstlinear source 1122 arranged to irradiate the back side of a plate 1114mounted on transparent surface 1112, such as a glass plate, and a secondlinear source 1120 arranged to irradiate the top side of the plate. Eachlinear source extends to cover one dimension of the plate, which in theexample shown shall be referred to as the transverse direction. Thecarriage traverses the plate in the longitudinal (or lateral) directionalong arrow L, with at least one source, and preferably both sources,activated. While the exposure step may be performed in a single pass, insome embodiments the exposure may be performed in a plurality of passes,in which each pass imparts radiation using both banks of sources at afraction of the total exposure needed to provide a desired amount ofexposure. As will be understood, the carriage may have a first speedwhen traversing the plate along the direction of arrow L with radiationsources activated, and a second, faster speed when traversing the platein the direction opposite arrow L, to reset for another pass or at thecompletion of the desired number of passes.

The overall mechanism for creating the exposure may comprise a tablehaving an outer frame 1110 that holds a transparent (e.g. glass) innerportion 1112. The upper 1120 and lower 1122 linear radiation sources(e.g. banks of LED point sources, optionally mounted inside a reflectivehousing) are mounted on a gantry system or carriage 1130. The radiationsources are connected to a power source, such as an electrical powercord having sufficient slack to extend the full range of motion of thecarriage. Tracks (not shown) disposed on the outer frame portion providea defined path for the gantry system or carriage to traverse. Thecarriage may be moved on the tracks by any drive mechanism known in theart (also coupled to the power supply and the controller), including achain drive, a spindle drive, gear drive, or the like. The drivemechanism for the carriage may comprise one or more components mountedwithin the carriage, one or more components fixed to the table, or acombination thereof. A position sensor (not shown) is preferably coupledto the carriage to provide feedback to the controller regarding theprecise location of the carriage at any given time. The control signaloutput from the controller for operating the radiation sources and forcontrolling motion of the carriage may be supplied via a wired orwireless connection. The controller may be mounted in a fixed location,such as connected to the table with a control signal cable attached tothe sources similar to the power cable, or may be mounted in or on thecarriage. The control system and drive mechanism cooperate to causeback/forth relative motion in a transverse direction between the lightfrom the radiation sources and the plate. If should be understood thatother embodiments may be devised in which the drive mechanism isconfigured to move the portion of the table containing the plate paststationary upper and lower linear radiation sources, as well asembodiments in which the radiation sources cover less than the fullwidth of the plate and are movable in both the transverse andlongitudinal direction to provide total plate coverage (or the plate ismovable in both directions, or the plate is movable in one of the twodirections and the sources are movable in the other direction toprovides the full range of motion required to cover the entire plate).

In one work flow configuration, the table for conducting the exposurestep (i.e. exposure table) as described above may be positioned toautomatically receive an imaged plate from an imager. For example, animager may be positioned so that the imaged plate expelled therefromlands in a first location, and a robotic handling device may beconfigured to automatically pick up and move the imaged plate from thefirst location to a processing location on the exposure table, where theexposure process as described herein is then performed using transverselinear sources attached to a carriage that traverses the platelongitudinally.

As discussed in U.S. Pat. No. 8,578,854 and illustrated schematically inFIG. 12, each bank 1200 of LED sources may comprise a plurality ofdiscrete units 1210 having a plurality of individual LED point sources1212 on each unit, with the plurality of point sources arranged in aplurality of lines 1220, 1222, 1224, 1226, 1228, 1230. All of the pointsources on each unit may be controlled together, may be individuallycontrolled, or may be controlled in groups. For example, each line ofpoint sources in each unit (e.g. each of lines 1220, 1222, 1224, 1226,1228, 1230) may be separately controllable. Providing such a fine levelof control may have several advantages. For example, the actual outputfrom each line of LEDs may vary slightly for the same amount of inputenergy, due to variations in the LEDs themselves, soldering to thecircuit board, cooling, decay or wear over time, and the like, and thus,each line of LEDs may be characterized and their intensity varied by anappropriate factor relative to other lines to so that the radiationoutput produced by each line is as close to homogenous as possible.Characterizations and re-calibration may be performed on a periodicbasis to account for variations in the lines over time. Suchcharacterizations may be performed by positioning a sensor that measuresincident radiation at a predetermined distance from each line of LEDsources. On top of compensation for variations in the output intensityof the LEDs themselves, further compensations may be made for variationsin transmissivity of any structure that lies between the sources and theprinting plate, such as for example, the glass surface 1112 that liesbetween the back sources and the printing plate in the configurationshown in FIG. 11. Any characterizable variations in transmissivity ofemitted radiation through the glass surface can be countered by varyingthe intensity of the LEDs based upon carriage location so that theamount of radiation that reaches the back of the plate is as close tohomogenous as possible over the entire exposed plate area.

Definitions

The term “single element structure number” as used herein refers to asquare defined by the total number of pixels that comprises that square.For example, a “64-pixel single element structure” comprises square 200,which comprises an 8×8 grid of pixels 202, and has a total of 64 pixels,as illustrated in FIG. 2A. Likewise “144-pixel single element structure”250 comprises a grid of 12×12 pixels 202, yielding a total of 144pixels, as illustrated in FIG. 2B.

The term “dot top diameter” refers to the diameter of the top of aprinting plate element or “dot” (i.e. the portion of the element thatcontacts the printing surface), as illustrated in FIG. 3A, showing aphotograph of a 3-dimensional perspective side view of an exemplaryprinting plate element 300 and its dot top diameter 310. The term “dotground diameter” refers to the diameter at the base of a printing plateelement or “dot” (i.e. the diameter of the element at the floor or“ground” of the plate), as illustrated in FIG. 3B, which is a photographof a top view of exemplary printing plate element 300 and its dot grounddiameter 320. The term “printed dot diameter” refers to the diameter ofthe dot that is printed on a substrate by a printing element, asillustrated in FIG. 3C, which is a photograph of a top view of printeddot 350 and its printed dot diameter 330.

Example

For optimization of the time delay, single element structures of varioussizes were imaged by a laser into the mask of a photosensitive printingplate at a resolution of 4000 dpi. For this example, a Model No. DPR 045printing plate, manufactured by DuPont, was used.

The photosensitive printing plates were then back exposed, such as byusing UV radiation source 120, and main exposed, such as by using UVradiation source 110, as depicted in FIG. 1. For this example, eachsource 120 and 110 source comprised a linear source comprising a bank ofindividual LED UV point sources, as described in more detail herein. Theplate was exposed in a single exposure step using a main side UVirradiance of 230 mw/cm² at a wavelength of 360 nm and a back side UVirradiance of 17 mw/cm² at the same wavelength at a relative plate speedof 1.25 mm/sec. For this example, the UV radiation sources were movedlengthwise under and above the surface of each photosensitive printingplate at the specified speed. The time delay was varied to optimize thesmallest single dot element on the processed photosensitive printingplate and printed to optimize the smallest printable dot size on theprinting substrate.

Results of exemplary time delays for exemplary single element structurenumbers 64 and 144 are shown in FIGS. 4-6. As shown in FIG. 5, a plot ofthe ground diameter of the smallest processed single dot element versusthe time delay between back exposure and main exposure for any set ofconditions yields a maximum 500 (i.e. 573.33 μm diameter at 92 secondstime delay, for the plot shown). Thus, the size of the base of the dot,and therefore the stability of the shape, can be optimized by optimizingthe time delay between back exposure and main exposure. As shown in FIG.6, a plot of the smallest printed dot on the substrate versus the timedelay between back exposure and main exposure yields a minimum 600 (29μm diameter at approximately 92 seconds time delay, for the plot shown).In general, the smallest printed dot size is desirable for highestresolution. In general, the smallest printed dot size with the largestdot ground diameter is optimal.

The optimized results shown in FIGS. 4-6 above are specific to theparticular printing plate system and other variables, such as speed,energy density, etc., for the example discussed herein. It should beunderstood to those of skill in the art that different printing platesystems, different speeds, different energy densities, and othervariables may impact the optimum results achievable by the processdescribed herein, and that similar graphs and optimums can be generatedfor any type of print system. In general, however, the delay timebetween the rear side exposure and the front side exposure may generallyfall in the range between 10 and 200 seconds, more preferably a rangebetween 2 and 100 seconds, and most preferably in a range of between 5and 20 seconds. Minimizing the delay time minimizes overall processingtime, and thus has an impact on overall throughput of a system.Accordingly, optimizing other conditions to minimize the time delay mayalso be beneficial.

Although the exemplary system shown in FIG. 1 illustrates the time delayschematically in a linear system, it should be understood that variousexposure systems may be devised to provide the optimized time delay. Insuch exemplary systems, the UV light sources may comprise, for exampleand without limitation, LEDs, arrays of LEDs, fluorescent lights, suchas fluorescent tubes, arc discharge lamps, or any other UV light sourceknown in the art. Although described herein in connection with a UVsystem and referring to “UV light”, it should be understood that thetechnology described herein is not specific to any particular type ofradiation wavelength, visible or non-visible, and that the system mayutilize any type of actinic radiation or other radiation that isfunctional to cause the photochemical reaction necessary to cure thetype of plate used. Thus, the term “light source” as used herein refersto any type of actinic radiation source.

In one embodiment 700 depicted in FIG. 7, the printing plate 730 may bemounted on a transparent (e.g. glass) cylinder 760 rotating at apredetermined speed, with the main radiation source 710 disposed in afirst location along the cylindrical path of rotation adjacent theexternal surface of the cylinder, and the back side radiation source 720disposed in a second location along the cylindrical path of rotationadjacent the internal surface of the cylinder, with the respectivelocations of the sources spaced apart by the distance required toprovide the time delay required at the speed of rotation. In such asystem, the location of the light sources and/or the speed of rotationmay be variable to provide different time delays. The photosensitiveprinting plate 730 may be a sleeve, such as a sleeve designed to fitover the transparent cylinder 760 of the system described above, or maybe flat, but sufficiently flexible, to permit it to be disposed on andsecured to the surface of the cylinder. It should be understood that theterm “transparent” as used herein may refer to any material that permitsa desired amount of radiation at the desired wavelength pass through theselected material. Thus, “transparent” as used herein, may refer to amaterial that is not visibly transparent or even translucent to thehuman eye.

In another exemplary embodiment 800, depicted in FIG. 8, each collectiveradiation source 810, 820 may emit a planar radiation field that is atleast coextensive with both lateral dimensions of plate 830 (e.g. eachcollective radiation source 810, 820 may be configured to irradiate theentire plate surface all at once when activated, if configured to beactivated in that manner), in which case the controller 850 may beconfigured to create a delay time by creating a time difference betweenturning on a portion of source 820 for exposing the back surface andturning on a portion of source 810 for exposing the main surface. Theprinting plate 830 may lay flat on a horizontal transparent (e.g. glass)plate 860 or may hang in a vertical orientation, such as from a hangar170 as depicted in FIG. 1. Although depicted schematically as singlecontinuous sources 810, 820 in FIG. 8, each source 810, 820 preferablycomprises a plurality of individual subsources (not shown), such asfluorescent tubes or LED point sources that are individuallycontrollable or controllable in subsets smaller than the overallirradiation field. The plurality of subsources may be coordinated andcontrolled to act as a single source, or individually activated in adesired pattern. For example, in a configuration comprising a pluralityof stationary subsources and a stationary plate, the individualsubsources may be independently controlled so that fewer than all of theindividual subsources comprising source 810 are turned on at the sametime and fewer than all of the individual subsources comprising source820 are turned on the same time. The collective subsources may thus becontrolled in any pattern that provides the desired time delay andavoids simultaneously irradiating the front and the back of the plate bysubsources that are spatially aligned with one another relative to thesame coordinates of the plate.

One exemplary control pattern may activate the radiation subsources in asequence that causes relative motion between the radiation field and theplate, such as a movement that essentially mimics the same lightpatterns that would be provided by main and back linear sources attachedto a carriage, but with the advantage of having no moving parts. Theillumination pattern may be configured to illuminate multiple portionsof the front and back simultaneously (e.g. such as in a pattern thatmimics multiple carriages—one starting at one end of the plate, and onestarting in the middle). The illumination pattern in such aconfiguration is not constrained to patterns that mimic one or morecarriages, however, and may be implemented in any pattern that providesthe desired time delay, overall exposure, and lack of simultaneousexposure from front and back for any particular cross sectionalcoordinate of the plate. The pattern may also comprise illuminating theentire back at once and then the entire front, either in a singleexposure for each side, or in fractional exposures of the full requiredexposure for each side, with the desired time delay applied between eachfront and back exposure. Furthermore, although shown in a flatconfiguration, it should be understood that systems in which both theplate and the sources are stationary may also be arranged in acylindrical configuration.

Optionally, the embodiment shown in FIG. 8 may also include optics (notshown). These optics may include lenses, mirrors and/or other opticalhardware components to direct and/or confine the radiation emitted fromthe plurality of individual subsources (e.g. LEDs) to a specific area onprinting plate 830. This configuration may produce a stronger contrastbetween the dark and illuminated areas on printing plate 830, therebyincreasing accuracy of the exposure process.

In yet another exemplary embodiment 900, a stationary plate 930 may besubjected to irradiation from a single linear source 915 that isconfigured to pass over both the front side and back side of the plateat a speed that provides the desired time delay, with a controller 940that, for example, may turn the source on and off at the appropriatetimes or modulate the amount of radiation between a main exposureintensity and a back side exposure intensity, as needed. The plate maybe disposed on a substrate 960 in a horizontal system, as depicted inFIG. 9, or the system may be oriented vertically, as described in otherembodiments. It should be understood that the source may travel ineither direction, so long as the controller first commences irradiationat the leading edge of the back side. The structure for moving thesource may comprise, for example, a holder for the source mounted to abelt or chain that moves in a desired path. The source may move at adifferent speed (e.g. slower) when aligned over one side of the plate tocause an exposure than it does when it is traveling between the trailingedge of one side and the leading edge of the other side. In mostembodiments, because the time delay is generally a fraction of theoverall exposure time needed to expose the plate, this embodiment may becommercially practical only in processes in which the total exposure isspread over multiple passes.

It should be understood that the invention is not limited to anyparticular physical embodiment, and that the method of the invention ofincorporating an optimized delay between back side and front sideexposure may be performed in any system having any physicalconfiguration.

FIG. 10 illustrates an exemplary method for preparing a printing platein accordance with the invention, including in step 1000, commencingirradiating the back side of the printing plate, implementing a definedtime delay in step 1100, and then, immediately at the end of the timedelay, commencing irradiating the front side of the printing plate instep 1200. In one embodiment, the exposure may be carried out using amultitude of consecutive exposure steps, in which each step contributesa fraction of the total energy dose required for complete curing of theplate, as is known in the art. In accordance with the invention,however, each exposure step includes the requisite time delay. Thus, asdepicted in FIG. 10, in such an embodiment, each radiation step 1000 and1200 may comprise only a fraction of the total radiation desired, andsteps 1000, 1100, and 1200 may be repeated until the plate has beenexposed to the total amount of radiation desired.

In a method for optimizing the time delay for a specific type ofprinting plate at a specific set of exposure conditions, alsoillustrated in FIG. 10, the method further comprises creating a firstsample in step 1300, performing steps 1000, 1100, and 1200 on the sampleat the specific set of exposure conditions, creating a printcorresponding to the sample in step 1400, changing the time delay instep 1500, and then creating a new sample in step 1300 and performingsteps 1000, 1100, and 1200 on the new sample. Steps, 1300, 1000, 1100,1200, 1400, and 1500 may be repeated in sequence for a plurality ofsamples as many times as desired. Then, in step 1600, the optimum timedelay is selected. In some embodiments, the time delay corresponding tothe print having the smallest print dots may be optimal. In others, theoptimum time delay may correspond to a minimum value for the smallestprinted minimum dot diameter that coincides with a maximum value for thedot ground diameter for a range of time delay values.

Notably, as illustrated in FIG. 1, front side radiation source 110 andback side radiation source 120 do not spatially overlap one another.Thus, in relative motion systems, in addition to distance D between theleading edge 122 of light source 120 and the leading edge 122 of lightsource 110 (which may be an adjustable distance), there is alsopreferably a distance (d) between the trailing edge 124 of light source120 and leading edge 112 of light source 110. In other words, asillustrated in FIG. 13, at no time is any specific cross-sectionalcoordinate A, B, or C on the plate being exposed from both the frontside and the back side simultaneously, and thus the apparatus as a wholeis configured to prevent simultaneous irradiation of any specific crosssectional coordinate on the plate. As shown in FIG. 13, showing asnapshot of a particular portion of the plate during a specific momentin time during exposure, section A of plate 1330, which has ablated mask1320 on a top layer thereof, is irradiated by top source 1310, section Bis not irradiated by either source, and section C is irradiated bybottom source 1312, but there is no cross sectional coordinatecorresponding to a line parallel to A, B, or C, that is beingsimultaneously irradiated by both sources. However, because the timedelay is a fraction of the overall exposure time for the plate, bothsources are actively providing radiation to some portion of the platesimultaneously over at least a portion of the exposure time in mostsystems for most plate sizes. By making distance D shown in FIG. 1adjustable, the relative motion velocity between the plate and thesources can be varied within a certain range, without changing the timedelay between back and front exposure, because within that range D canbe adjusted to compensate for the change in relative velocity.

Such a configuration may be provided by a spatial configuration asdepicted with respect to FIG. 1, by a configuration of the controller,or by a combination thereof. Thus, in a system that does not create thetime delay using a spatial distance between the main and back sideradiation sources in combination with relative movement, but rather bypulsing stationary sources relative to a stationary plate, such as isdepicted in system 800, back side radiation source 820 (or one or moresubsources) may spatially overlap with front side radiation source 810(or one or more of subsources), but the controller is configured so thatsuch overlapping sources never actively irradiate the plate at the sametime.

Finally, while the time delay may be the same for each area of theplate, it should be understood that depending upon the configuration ofthe radiation sources, controller, and control scheme, one portion ofthe plate may be irradiated differently than another, if desired.

Processes for Increasing Back-Exposure

As described above, photo polymer printing plates typically need UVexposure from the front side and the back side. The front side exposureis applied through the mask, which holds the image information thatshall be imposed to the printing plate. The rear side is exposed to UVradiation through the rear side plastic substrate without any vignettingin order to build a polymerized support layer over the entire plate forthe fine printing details located on the plates front side. Thispolymerized support layer is called “floor.” The floor thicknessdetermines the relief depth for a given plate thickness.

Recent advances in technology for curing of photopolymer printing plateswith UV light has produced a variety of LED-based exposure units fromvarious suppliers. These units comprising LED UV light sources areincreasingly replacing so-called “bank” exposure devices thatincorporate fluorescent tube technology, in which light from thefluorescent tubes typically covers the complete plate surface at onetime. Because LEDs are more expensive and require more complex driverelectronics, while delivering a much higher UVpower-per-unit-surface-area than fluorescent tubes, many embodimentscomprise a light source that covers only one dimension of the polymerplate completely (e.g. width), while using relative motion between theplate and UV head to cover the second dimension (e.g. length), to ensureall surface areas of the plate are exposed to the UV light. One exampleof a state of the art exposure system is the XPS 5080 systemmanufactured by ESKO. This exposure system is equipped with 3 identicalUV heads: two for front side exposure, and one for rear side exposure.

Thus, in one embodiment of the invention, the exposure system comprisesthree (typically identical) UV heads (e.g. linear sources) comprising aplurality of LED sources: two heads directed to the front side (alsoreferred to herein as the “main exposure heads”), and one head directedto the rear side (also referred to herein as the “back exposure heads”),as shown in FIG. 1B. Each UV head covers only one dimension of the plate(e.g. width) completely, with reliance on relative motion between thesource and the plate to cover the other plate dimension (e.g. length),as described in other embodiments shown and described herein (see, e.g.,general configuration of the linear sources in FIG. 11).

Using the apparatus and method described above may present a challengewith respect to curing photopolymer printing plates having back sidesthat are less sensitive to UV radiation. Such lesser sensitivity mayarise from relatively higher photo-blocker content in the platerear-side plastic substrate. One way of overcoming this challenge is toapply back exposure energy to the plate rear side prior to the combinedback-main exposure steps.

For common standard polymer plates, the ratio (Rbf) between thepredetermined amount of energy applied to the rear (back) side andenergy applied to the front side of the plate to achieve desiredresults, ranges according to plate supplier information from 1:5 to1:40, respectively 20% to 2.5%, depending on the plate type and therelief depth to be achieved.

A standard polymer printing plate, such as for example, the DuPont®model DPR 045 plate, may require approximately 12 minutes of frontexposure and about 60 Seconds of back exposure for complete curing on abank exposure unit. In an LED UV exposure unit configured such as thosedescribed herein, and in particular in the XPS 5080 system manufacturedby Esko-Imaging Graphics GmbH, Rbf may be as high as 16%.

In certain highly-UV-sensitive plates, such as EXS model plates fromDuPont or FTF plates from Flint, a back exposure of only about 6 secondswould optimally be required using such systems, if the plates were nototherwise adjusted. As most bank exposure units control exposure time in1-second increments, adjusting the floor thickness with back exposurefor such high sensitive plates would be very difficult to achieve,without making some adjustments to the plate structure. Hence, platesuppliers now add a higher content of UV blocker into the rear sideUV-transmissive, dimensionally-stable plastic substrate of the plate,which blocks a substantial amount of UV from participating in the curingprocess from the rear side. Such adjustments by plate suppliers havepushed Rbf closer to, for example, approximately 50%.

Unfortunately, if the content of UV blocker inside the rear side plasticsubstrate is not controlled very precisely by the plate suppliers andvaries from one plate charge to another, it may be necessary to controlthe resulting floor thickness of the plates periodically by readjustingthe back exposure UV-energy accordingly.

Moreover, in many configurations, the plate typically lies flat on aglass table with its rear side adjacent the glass plate and isback-exposed through this glass plate. The glass plate typically has aUV transmission of around 80%, consuming 20% UV, thereby pushing the Rbfas high as, for example, approximately 62.5% for the exemplary platesdescribed above.

Another demand for increase of back exposure power arises from the factthat the exposure may be applied to the plate in several fractions.Using several smaller exposure-energy factions, the curing becomes lessefficient, making the total exposure energy required for complete curinghigher in comparison to an exposure applied in an uninterrupted step.The loss of efficiency due to using multiple exposure steps results inanother increase in Rbf, bringing it close to, for example,approximately 75% for the exemplary plates described above.

An embodiment equipped with three identical heads—one UV head for backexposure and two UV heads for main exposure—as depicted in FIG. 1B, hasan inherent Rbf of 50%, while the back exposure requirements for ahighly sensitive plate having the combined characteristics noted above,may require an Rbf of 75%.

It is thus not possible to adjust the floor thickness exclusivelythrough irradiance of the rear side UV Head using three identical headsoperating at the same power output, and for reasons described hereinlater, it may be desirable to use three identical heads with the samenominal power operating at that nominal power, for greatest efficiencyand productivity. One method for supplying the missing 25% of backexposure UV power is to provide a back-only exposure step to provide theadditional energy in an uninterrupted exposure into the rear side of theplate before the combined main and back exposure steps are commenced.

As shown in FIG. 1B, three identical UV heads are disposed relative tothe photopolymer plate to be cured. The plate comprises a mask 2132, aphoto sensitive polymer 2134 and a plastic substrate 2136 placed flat ona glass plate 2160, with the non-printing side of the plate in contactwith the surface of glass plate. Two heads for the main exposure 2110,which cure the front side of the polymer printing plate, are locatedabove the glass plate and the polymer plate thereon, and the third UVhead for back exposure 2120 is located under the glass plate 2160. Allparameters, like speed of the UV sources, distance between UV sources,the resulting time delay between the UV Source, the irradiance of theSources as well as the number of exposure cycles is controlled by thecontrol system 2140.

A standard exposure process comprises at least two exposure steps,wherein in each step the exposure heads move from a start position alongthe polymer plate, exposing the plate to actinic UV radiation withconstant speed V in the direction of the arrow shown in FIG. 1B, andafterwards returning in the direction opposite the arrow to their startposition without emitting radiation in the return pass. During thisprocess, in accordance with aspects of the invention described herein,the back exposure is applied to the polymer plate before the mainexposure by a precisely determined time difference. Accordingly, theback exposure head moves a constant distance in advance of the mainexposure head during polymer plate exposure, causing a constant delaytime between rear and front exposure. The distance between main UV headsand back exposure head, and consequently the delay time, is typicallyadjustable.

As described herein, total exposure time is determined by UV outputaperture of the UV heads in traveling direction, the speed by which theheads travel along the polymer plate and the number of passes the headmoved along the plate. The width of the UV output aperture divided bythe traveling speed results in the time that a pixel in the plate “sees”UV light. This time is called “pixel time” in the following text.

The intent of the foregoing method is to obtain a cure of the polymerplate that is superior to a simple front and backside exposure eachapplied in only one uninterrupted step. “Superior” in this case meansthe plate holds smaller printing details, which are fixed to the platefloor with higher stability and that do not bend during printing.Producing smaller print dots enables production of lighter highlights inthe print.

The back exposure energy is increased or decreased to adjust the floorthickness of the plate. This can be done either by adjusting theirradiance of the back exposure UV head, by adjusting the pixel time, orby adjusting both pixel time and irradiance. Due to the nature of thecuring process, changing the pixel time is disfavored, as it can alsoaffect the front exposure results, which may lead to unwanted curing andprinting results. Thus the preferred method for adjusting the platefloor thickness is to change the irradiance.

Productivity is a highly desirable characteristic of UV exposuredevices, and thus it is favorable to run the UV light sources at thenominal UV output in order to supply the energy to the plate requiredfor curing as fast as possible. As a consequence, both front exposure UVheads in the system depicted in FIG. 1B are preferably normally operatedat nominal UV power. Depending on the energy dose ratio Rbf between backand front side exposure, it is not always possible to reach the maximumproductivity, such as for example when the required back exposure energyis 75% of the energy required for front exposure, and the one head onthe back side can only deliver 50% of the front exposure energy. On theother hand, equipping the back side exposure with more UV heads isdisfavored, as this increases system manufacturing costs, because headscan only be added in integer numbers, which may cause too much rear sideUV power for most plates.

In the example discussed above, where the irradiance of the backexposure head cannot be increased any further, absent some other remedy,the UV output of the back exposure would have to be set to maximum andthe pixel time increased by 50% to reach the required floor thickness(with some corresponding change to the front exposure to adjust for theincreased time). This approach may affect the quality of the frontexposure curing result, and thus, it may be necessary to evaluate thecomplete set of exposing parameters to maintain desirable plate printingquality.

Alternatively the number of exposure cycles may be increased until theaccumulated rear side exposure energy is sufficient to cure the platefloor to the required thickness. But this approach will also increasethe front exposure energy, leading to higher energy input if the frontexposure irradiance stays the same as before the increase in the numberof cycles, which may lead to different curing and printing results. Alsoif the irradiance of the front exposure is reduced to maintain the frontexposure energy the same as before the increase of the number ofexposure cycles, this may lead to different curing and printing results,making this an imperfect solution.

Table 1 below provides a survey of different Rbf ratios for variousexemplary plate types. As shown in Table 1, the Rbf ratio increases forthicker plate materials. For thicker plate material and also for the newhighly-UV-sensitive materials, like the EXS plate from DuPont or the FTFplate from Flint, there is a need for further improving the efficiencyof the XPS exposure device. The plates listed in the table are merelyexamples, and the invention is not limited only to use with digitalflexographic printing plates listed.

TABLE 1 UV back side UV main side Ratio energy no of energy Back/ backside Irradiance speed cycles main side Irradiance Front Plate type[mJ/cm²] [mW/cm²] [mm/s] [n] [mJ/cm²] [mW/cm²] Rbf [%] DRAVE 045 3944136 9.9 4 9019 311 43.7 DRAVE 067 4350 150 9.9 4 9019 311 48.2

One preferred method for bringing more back side exposure energy intothe plate, without sacrificing the desired printing results of the platederived from the proper selection of exposure parameters, comprisesexecuting one or more additional back exposure steps prior to theconsecutive combined back and front exposure steps discussed herein.This additional back exposure step is preferably applied directly beforestarting the consecutive exposure steps.

Thus, for the processes described and depicted relative to FIG. 10, oneor more back-only exposure steps for adjusting the plate floor may besequentially performed immediately prior to step 1000. This methodeliminates any need for new time-consuming parameter evaluation of allparameters in the event of needing to adjust the floor. For example, inprocesses that include step 1300-1600 for characterizing a plurality ofsamples, one or more back-only exposure steps may be included to set thedesired amount of floor as part of the initial characterization of aparticular type of plate. Then, in the event a particular batch ofplates may require adjustment of the floor relative to the originalamount of back-only exposure characterized for that type of plate, theadjustment process may include only adjusting the back-only exposureamount, keeping all other parameters in all other steps the same. Whiledetermining the correct adjustment for the floor may include aniterative process of generating and evaluating one or more samples withvaried amounts of back-only exposure, adjustment of the floor may alsobe effected by merely increasing or decreasing the back-only exposureproportionally relative to the increase or decrease in floor desired.

The energy applied by the additional back exposure steps may becontrolled by the exposure time, namely by the speed by which the UVhead is moved along the plate rear side. In order to keep the timeperiod for this additional exposure step as short as possible, the UVirradiance of the rear exposure UV head is ideally operated at thenominal maximum.

The amount of additional back exposure UV-energy may be controlled bycontrolling the irradiance of the back exposure UV-heads, by adding moreback exposure steps (each step comprising a fractional amount of a totalthat is predetermined to be required to provide the desired floorthickness adjustment), or by some combination of irradiance, exposuretime (the speed by which the UV head is moved along the polymer platerear side), and the number of additional exposure steps.

Additional back exposure steps are added whenever the nominal backexposure irradiance is not sufficient for complete curing of the floorto the required thickness. The additional back exposure step or steps(as well as any and all of the method steps described herein) may beimplemented via the apparatus controller, which controller may be in theform of computer hardware programmed with software instructions forcausing the components of the apparatus to perform the subject steps.Although shown and described in connection with a certain embodiments ofthe invention described herein, it should be understood that exposureprocesses comprising additional back-exposure steps are not limited toany particular process or apparatus. Similarly, although the use ofadditional back-only exposure steps may be particularly suitable for UVexposure systems, this aspect of the invention is not limited to anyparticular radiation system.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

What is claimed:
 1. An apparatus for preparing a printing platecomprising a photosensitive polymer activated by exposure to radiation,the printing plate having a non-printing back side and a printing frontside with a mask for defining an image to be printed, the apparatuscomprising: a plurality of radiation sources, comprising at least onefront source positioned to expose the front side of the plate toradiation, and at least one back source positioned to expose the backside of the plate to radiation, the at least one front source and the atleast one back source each having an irradiation field covering an areaat least coextensive with a width of the plate, the irradiation field ofthe linear front source and the irradiation field of the linear backsource spaced apart from one another by a lateral distance along thelength of the plate; a holder configured to receive the printing platein a position to receive incident radiation from the plurality ofradiation sources; a controller connected to the plurality of radiationsources; means for causing relative movement between the printing plateand the at least one front source and the at least one back source; theapparatus configured to, for each specific coordinate corresponding to across-sectional portion of the plate, first (a) commence irradiating theback side of the plate, then (b) automatically impose a preciselydefined and repeatable time delay, and then (c) immediately after thetime delay elapses, commence irradiating the front side of the plate,without exposing any specific coordinate to front side and back sideirradiation simultaneously, by providing relative movement between theplate and the irradiation fields from the at least one front source andthe at least one back source at a velocity sufficient to cause thedefined time delay over the lateral distance.
 2. The apparatus of claim1, wherein the apparatus is further configured to adjust floor thicknessof the printing plate by providing one or more back-side-only exposuresteps prior to performing step (a), each back-side-only exposure stepcomprising providing relative movement between the plate and theirradiation field from the at least one back source.
 3. The apparatus ofclaim 1, wherein the one or more radiation sources comprise a pluralityof LED point sources configured to emit UV light, the plurality of LEDpoint sources collectively configured to illuminate a collectiveillumination area, the LED point sources controllable by the controllerover an output area that is less than the collective illumination area,wherein each LED point source is a member of a group comprising aplurality of LED point sources, and the controller is configured to (a)provide individual control of light intensity from each group tocompensate for variations in light output of one group relative toanother, (b) compensate for variations in transmissivity of a surfacedisposed between the light source and the plate, or (c) a combinationthereof.
 4. The apparatus of claim 1, wherein the apparatus isconfigured to provide radiation from the at least one front source andthe at least one back source simultaneously over at least a portion of atotal plate exposure time.
 5. The apparatus of claim 4, wherein atrailing edge of the irradiation field of the back source is spacedapart by a lateral distance along the length of the plate from a leadingedge of the front source.
 6. The apparatus of claim 5, wherein theprinting plate is fixed and a drive mechanism is configured to move thefront and back radiation sources.
 7. The apparatus of claim 6, whereinthe holder comprises a substrate on which the plate is disposed, whereinthe substrate is at least partially transparent to the radiation.
 8. Theapparatus of claim 7, wherein the means for causing relative movementcomprises a carriage to which the front and back sources are mounted andlaterally spaced apart from one another.
 9. The apparatus of claim 8,comprising two front sources and one back source.
 10. The apparatus ofclaim 9, wherein each of the two front sources and one back source areidentical.
 11. A process for exposing a printing plate comprising aphotosensitive polymer activated by exposure to radiation, the printingplate having a non-printing back side and a printing front side with amask for defining an image to be printed, the method comprising for eachspecific coordinate corresponding to a cross-sectional portion of theplate, the sequential steps of: (a) commencing irradiating the back sideof the printing plate; (b) automatically imposing a precisely definedand repeatable time delay; (c) immediately after the time delay elapses,commencing irradiating the front side of the printing plate, withoutexposing any specific coordinate to both front side and back sideirradiation simultaneously.
 12. The process of claim 11, wherein themethod further comprises adjusting floor thickness of the printing plateby providing one or more back-side-only exposure steps prior toperforming step (a).
 13. The process of claim 11, further comprisingrepeating steps (a) through (c) using a fractional amount of a totalradiation exposure in each radiation step (a) and (c) until the platehas been exposed to a desired amount of total radiation.
 14. The processof claim 11, comprising providing radiation from the at least one frontsource and the at least one back source simultaneously over at least aportion of a total plate exposure time.
 15. The process of claim 11,comprising exposing a plurality of printing plates identical instructure except for differing content embodied in their respectivemasks, wherein the exact same precisely defined and repeatable timedelay is imposed in step (b) for each of the plurality of plates. 16.The process of claim 11, comprising performing the process using anapparatus comprising one or more radiation sources configured to exposethe front side and the back side of the printing plate to radiation anda controller connected to the one or more radiation sources andconfigured to commence irradiating the back side of the plate first andto commence irradiating the front side of the plate after the definedtime delay, the apparatus comprising at least one linear front sourceand a linear back source spaced apart from one another by a lateraldistance along the length of the plate, the process comprising causingrelative movement between the printing plate and the at least one linearfront source and the linear back source, wherein the relative movementhas a velocity sufficient to cause the defined time delay over thelateral distance.
 17. The process of claim 16, wherein the methodfurther comprises adjusting floor thickness of the printing plate byproviding one or more back-side-only exposure steps prior to performingstep (a), and energy applied to the photopolymer printing plate in theback-side-only exposure step is controlled at least in part by:controlling irradiance of the linear back radiation source; controllingspeed of the linear back radiation source movement; controlling a numberof consecutive back-side-only exposure steps of the linear radiationsource, each back-side-only exposure step comprising a fractional amountof a total radiation exposure required to provide a desired amount offloor thickness adjustment; or a combination thereof.
 18. A printingplate produced by the process of claim 11, wherein the printing platehas stable print dots smaller than a plate prepared using a process thatdoes not include the precisely defined and repeatable time delay betweencommencement of back side and front side irradiation.
 19. A printingplate produced by the process of claim 11, wherein the printing platehas a floor depth greater than a plate prepared using a process thatdoes not include the back-only exposure step prior to commencing thestep of providing combined back side and front side irradiation.
 20. Anapparatus for preparing a printing plate comprising a photosensitivepolymer activated by exposure to radiation, the printing plate having anon-printing back side and a printing front side with a mask fordefining an image to be printed, the apparatus comprising: a pluralityof radiation sources, comprising a plurality of stationary front sourcespositioned to expose the front side of the plate to radiation, and aplurality of stationary back sources positioned to expose the back sideof the plate to radiation, the plurality of front sources and theplurality of back sources each having an irradiation field covering anarea at least coextensive with a length and width of the plate; a holderconfigured to receive the printing plate in a stationary position toreceive incident radiation from the plurality of radiation sources; acontroller connected to the plurality of front and back radiationsources, the controller configured to, for each specific coordinatecorresponding to a cross-sectional portion of the plate, first (a)commence irradiating the back side of the plate, then (b) automaticallyimpose a precisely defined and repeatable time delay, and then (c)immediately after the time delay elapses, commence irradiating the frontside of the plate, without exposing any specific coordinate to frontside and back side irradiation simultaneously, the controller configuredto adjust floor thickness of the printing plate by providing one or moreback-side-only exposure steps prior to performing step (a).